Sensors for detecting hazardous gases are well known for monitoring potentially hazardous environments, such as mines and industrial facilities that use or produce combustible and other hazardous gases. These sensors must be exposed to the gas on a regular basis to ensure that the sensor is properly tested. This also helps to prevent annoying and costly false alarms, and failure to alarm when the gas reaches a dangerous level.
These sensors are tested by passing a gas of known fixed composition, from a supply, such as a compressed gas bottle, at a predetermined flow rate. Often a worker must visit each sensor within the facility and perform the testing at the sensor. If a supply of gas is not permanently provided at each sensor, the worker also must transport the gas, which by definition is hazardous. Most sensors require testing with both a span gas and a zero gas. A span gas is a gas containing a given amount of an impurity, the gas for which the sensor is designed to detect. A zero gas is a purified gas that has an impurity concentration below the minimum detection limit of the sensor.